a. Field of the Invention
The invention relates to metrology instruments for measuring surface profiles of flat articles and more particularly, to a recording surface profile measuring instrument for scanning flat articles having steps of various heights, such as in planar integrated circuits.
B. Prior Art
In the manufacture of semiconductor integrated circuits, it is common to deposit circuit structures on planar substrates, such that the circuit, when magnified, appears as a series of mesas of various heights rising above a plain. The performance of semiconductor integrated circuits relies, to an extent, on the dimensions of the substrate and the materials deposited thereon. It is customary to measure these dimensions in the circuit manufacturing process and to adjust variables to achieve desired integrated circuit dimensions. Of special interest are the vertical dimensions of these structures which cannot readily be measured by optical or scanning electron microscopes.
In many instances, the vertical dimensions in semiconductor integrated circuits are only a few hundred to few thousand angstroms. This requires a magnification of one thousand to one million times for measurement, while horizontal magnifications of only fifty to a few hundred times are required.
Surface profile measuring instrument measure such vertical dimensions by sensing them with a stylus which generates an electrical signal corresponding to the vertical profile taken along a line. A stylus of a prior art recording surface profile measuring instrument is typically attached to a shank and this shank is connected to the moving slug of a linear position differential transformer. The stylus, the shank and the slug must be suspended to allow motion of the stylus perpendicular to the scanned surface as it scans said surface.
Instruments of the prior art use springs to suspend the mass of the stylus and slug. A problem with this type of suspension is that vibrations transmitted to the instrument from a base, e.g. a table on which the surface profile measuring instrument is standing cause the spring-suspended stylus assembly to swing and oscillate. These swings and oscillations are then reflected in the output signal as noise. Prior art instruments need be placed on blocks of granite or on shock-insulation tables.
A problem of the prior art related to the disparity between the vertical and the horizontal magnifications, is that as a stylus scans the surface to be measured, any incline in this surface is grossly exaggerated on the output display of the surface profile measuring instrument. Thus, to display a measured step in such a manner as to obtain a horizontal recording of its bottom and its top, the table holding the substrate must be accurately mechanically levelled, i.e., made parallel to the stylus scan, before a measuring scan is performed. To obtain an output record with the line representing the flat plane of a planar substrate having a slope less than 5%, the substrate table has to be levelled better than the ratio of vertical-to-horizontal magnification, typically less than 0.0005% slope. Such levelling done manually is extremely time consuming. To manually level better than the example cited is not practically possible.
Another problem related to scale is that of the measurement falling off scale. If, say, an 800 Angstrom mesa or step is to be measured on a 1,000 Angstrom scale, the substrate carrying such a step has to be mechanically so positioned with respect to the measuring device, that the bottom of the step falls to within the first 200 Angstroms of the scale. If this requirement is not satisfied, a part of the step will fall outside the scale of the instrument, thus making measurement impossible. To position an object mechanically with such accuracy is very difficult.
An object of the invention has been to devise a suspension of all moving parts of a surface profile measuring instrument as to reduce and even eliminate effects of shocks transmitted from outside of the instrument, thus eliminating the need for special tables and also increasing the signal-to-noise ratio.
Another object of the present invention has been to devise a surface profile measuring instrument which would perform levelling automatically through the use of servo-controlled mechanisms. A further object has been to devise circuits which would accept the electrical signals from a scanner and process the signals in such a way as to permit a distinction between signals caused by sharp features of scanned surface profiles and signals caused by warpage of scanned surface, and which would suppress the latter.
Still a further object of the present invention has been to devise a surface profile measuring instrument which does not require precise positioning of specimens to be measured, with respect to the instrument, yet with the measurement to be made falling within the scale of a recording device.